Organic light emitting devices (OLEDs), which make use of thin film materials that emit light when excited by electric current, are becoming an increasingly popular form of flat panel display technology. There are presently three predominant types of OLED construction: the "double heterostructure" (DH) OLED, the "single heterostructure" (SH) OLED, and the single layer polymer OLED. In the DH OLED, as shown in FIG. 1A, a transparent substrate 10 is coated by an anode layer 11. A thin (100-500 .ANG.) organic hole transporting layer (HTL) 12 is deposited on the anode 11. Deposited on the surface of the HTL 12 is a thin (typically, 50 .ANG.-500 .ANG.) emission layer (EL) 13. The EL 13 provides the recombination site for electrons injected from a 100-500 .ANG. thick electron transporting layer 14 (ETL) with holes from the HTL 12. Examples of prior art ETL, EL and HTL materials are disclosed in U.S. Pat. No. 5,294,870, the disclosure of which is incorporated herein by reference.
The device shown in FIG. 1A is completed by the deposition of metal contacts 15, 16 and a top electrode 17. Contacts 15 and 16 are typically fabricated from indium or Ti/Pt/Au. The electrode 17 is often a dual layer structure consisting of an alloy such as Mg/Ag 17' directly contacting the organic ETL 14, and an opaque, high work function metal layer 17" such as gold (Au) or silver (Ag) on the Mg/Ag. When proper bias voltage is applied between the top electrode 17 and the contacts 15 and 16, light emission occurs from the emission layer 13 through the substrate 10.
The SH OLED, as shown in FIG. 1B, makes use of multifunctional layer 13' to serve as both EL and ETL. One limitation of the device of FIG. 1B is that the multifunctional layer 13' must have good electron transport capability. Otherwise, separate EL and ETL layers should be included as shown for the device of FIG. 1A.
A single layer polymer OLED is shown in FIG. 1C. As shown, this device includes a glass substrate 1 coated by an anode layer 3. A thin organic layer 5 of spin-coated polymer, for example, is formed over the anode layer 3, and provides all of the functions of the HTL, ETL, and EL layers of the previously described devices. A metal electrode layer 6 is formed over organic layer 5. The metal is typically Mg or other conventionally-used low work function metal.
"Full-color emission" can be achieved by devices such as the stacked device shown in FIG. 1D. The device in FIG. 1D includes blue, green and red OLED devices (20, 30, and 40, respectively) for the emission of blue, green and red light, and any combination thereof. It is often difficult to manufacture devices such as that shown in FIG. 1D because the deposition of multiple electrodes (50 and 60) within the stack is often problematic from a processing standpoint and often results in damage to the surrounding organic layers.
To avoid the potential problems associated with multiple electrodes within OLED stacks, full-color emission has been achieved by placing multiple, single-OLED stacks in a side-by-side configuration within a single pixel as shown in FIG. 1E. In the example shown in FIG. 1E, blue, green and red OLEDs 71, 72 and 73 are placed side-by-side on a common substrate 10 and make use of a common electrode 75. Although this configuration minimizes the use of electrodes, the space required for multiple stacks along the substrate 10 results in a degradation in display resolution.